U.S. Pat. No. 5,572,560 discloses a nuclear fuel assembly comprising a bundle of fuel rods with a spacing between the fuel rods varying along the length of the fuel assembly from a uniform pitch at the bottom part of the fuel assembly to a non-uniform pitch at the top part of the fuel assembly to allow accommodation between the fuel rods of a water channel having an increased cross-section in the top part of the nuclear fuel assembly.
Provision of the water channel having an increased cross-section in the upper part is intended to enhance neutronic efficiency in terms of equalized water-to-fuel ratio for all fuel rod positions.
However, critical power performance is not considered and may be hindered by such arrangement.